How a dust collection cyclone works

[James Gunning]

After reading lots of posts here and seeing the questions that are repeatedly asked, perhaps a new thread with a layman's explanation of how a cyclone functions might be helpful. After lots of research over the years, I have come to understand this is how a cyclone works. I hope the real experts can weigh in and correct any mistakes I make.

Here is a simple diagram of a cyclone like we use in a home woodworking shop. The dust laden air is drawn in by vacuum and enters on the right. The entry is not in the center of the cyclone, but offset all the way to one side of the cyclone body. The air follows the inside wall of the cyclone and begins to spin. As it spins, being heavier than the air, the dust is forced to the outside. The dust starts to be drawn down by gravity and follows the cone. Just like when a skater pulls in her arms and spins faster, the dust is kept to the outside of the cone and spins faster as the cone narrows, taking even lighter particles with it. Finally the dust drops into the bin at the bottom by gravity. The air slows toward the bottom of the cone, reverses flow, and is drawn up through the center tube. As the air slows and starts back upward, it drops more dust. Finally, the air passes through the blower and out to the filter, shown on the left side. Hopefully, the filter catches the finest particles that make it through the cyclone and exhausts clean air back into the room. Venting to the outside instead of a filter is often mentioned here. Just picture the pipe into the filter going outside with no filter on it.

I've tried to skip most of the more exact physics discussion in favor of the simplest explanation.

313642

[Steve Peterson]

Thanks for the explanation James. It makes perfect sense to me now. I always wondered if the cyclone was tuned to a specific particle size. The variable diameter appears to work for a broad range of particle sizes.

It would seem that a taller cone would provide more rotations and allow more changes for the dust to be separated. However, if it gets too tall then the exit air would be pulled from only half way down. So it must be a balance to find a cone that is as tall as possible and still pulls the exit air from the bottom. I wonder if a long center tube could force the air to be pulled from the bottom and allow extra height for the cone.

Another thought is that the cyclone operation requires lots of air to keep the rotation speed high enough for the best separation. I have a ClearVue 1800 with a 6" inlet. Most tools have a 6" port or two 4" ports. Some of my small sanders have 2.5" ports and I usually open a second 2.5" port to keep the airflow high enough on the 4" branches. Maybe I should also open a second 4" port to keep the airflow higher into the cyclone.

Steve

[William Payer]

I do not profess to have a lot of knowledge on how exactly a cyclone works, do not ignore the volume of the "cone" in relation to the ducting entering it. As we all know dust particles and wood shavings/chips are carried in the ductwork as long as a minimal air stream velocity is maintained. Too low velocity and the heavier particles are the first to drop out of the flow of air and settle in the duct (read "clog") As the air enter the cyclone body from a relatively smaller duct, its velocity drops suddenly, resulting in wood and dust particles to drop out of the sir stream , downward under the influence of gravity. (This explains the operation of those inexpensive garbage can chip collectors sold at various woodworking stores). If the air stream were not slowed like this the dust would simply stay in suspension and flow out the exit and accumulate in the filter.

[Jack Burgess]

After reading lots of posts here and seeing the questions that are repeatedly asked, perhaps a new thread with a layman's explanation of how a cyclone functions might be helpful. After lots of research over the years, this is how I have come to understand a cyclone works. I hope the real experts can weigh in and correct any mistakes I make.

Here is a simple diagram of a cyclone like we use in a home woodworking shop. The dust laden air is drawn in by vacuum and enters on the right. The entry is not in the center of the cyclone, but offset all the way to one side of the cyclone body. The air follows the inside wall of the cyclone and begins to spin. As it spins, being heavier than the air, the dust is forced to the outside. The dust starts to be drawn down by gravity and follows the cone. Just like when a skater pulls in her arms and spins faster, the dust is kept to the outside of the cone and spins faster as the cone narrows, taking even lighter particles with it. Finally the dust drops into the bin at the bottom by gravity. The air slows toward the bottom of the cone, reverses flow, and is drawn up through the center tube. As the air slows and reverses direction, it drops more dust. Finally, the air passes through the blower and out to the filter, shown on the left side here. Hopefully, the filter catches the finest particles that make it through the cyclone and exhausts clean air back into the room. Venting to the outside instead of a filter is often mentioned here. Just picture the pipe into the filter going outside with no filter on it.

I've tried to skip most of the more exact physics discussion in favor of the simplest explanation.

313642

James, thanks for the imfo. New to this and not sure where the vacumn source is (I assume it is at the top) where does that exhause go? Also the cyclone machines I understand (if not vented to outside) emit 5 micron size particles and below. the serious ones we breath. Still working on how to vent mine.

Jack

[James Gunning]

Steve,

Actually, in industrial applications, the cyclones are tuned to the type of material being handled. Diameter, length of cone, length of center tube, other types of internal air flow, etc. are used to optimize the cyclones efficiency. In our case as woodworkers, we are all putting virtually the same material in and our design needs are very similar. If you have a Clearvue, you likely have the best design on the market available at a reasonable price. You do want to have more airflow than can be gotten through a 4" duct. Ideally, you would have 6" duct from the machine all the way to the cyclone inlet. Opening a gate on another machine will let your impeller flow more air, and might help a little with the static pressure in your system, but probably won't help capture more dust where you are working. Our dust collection systems are low pressure-high flow systems. A shop vac is the opposite: high pressure-low flow. The shop vacs work OK with smaller hose, such as 2.5", the dust collectors need much larger, 6" or even more in some systems.

I do use an Oneida Dust Deputy with my shop vacs for ROS and some other hand operations where I want dust/chip pickup. In that scale, the 2.5" hose works fine with the small Dust Deputy cyclone. I also have a Rigid desk top sander and I use both the shop vac/Dust Deputy hooked up to the Rigid and my SDD cyclone with the hose positioned to help catch anything escaping above the table.

EDIT: To answer your question about the length of the center tube. I don't have the sources at hand, but as I recall, the optimum length of the center tube should be about the diameter of the cyclone. That seemed like it would be too short to me as well, but there it is. As one example, my Oneida SDD has the central tube length at about the diameter of the cyclone. When its operating, there are really two vortex systems in the cyclone. The first is what I described where the flow is sticking to the inside wall of the cyclone. The second is a vortex going up the center of the cyclone and into the central tube. Apparently a longer center tube would interfere with the airflow in the center.

[James Gunning]

Jack,

The vacuum source is the blower on top. Most are usually just a radial impeller (material handling type) mounted directly on a motor shaft, although they are belt driven in larger applications. The impeller is mounted off center in a scroll housing. The air is drawn up the center tube into the center of the impeller, then forced out the outlet. Since the cyclone and chip bin are supposed to be an air-tight sealed system, that creates a vacuum at the inlet that causes the airflow into the cyclone. The exhaust goes into a filter (pictured on the left in the drawing). It can be vented outside without using a filter. That is the best option if things like offending your neighbors, make-up air and not drawing combustion gases from gas water heaters and the like can be sorted out.

Some cyclones work better in different applications. Generally, our requirements are very similar. The size of particles the cyclone passes will vary with the actual design of the cyclone, how full your chip bin is, how clean your filters are, and what you are doing (planing, BS, TS, sanding). The notable example is if you have a large belt or drum sander. Those will produce more fines and you will want better filters and be forced to clean them more often.

[Jim Andrew]

I have a Aget Dustkop, bought it last summer, problem it has a 12 1/2" impeller. The dust collector is a push through, like a bag type dust collector, although it is a cyclone. My idea is to purchase a 5 hp motor, and 16" impeller from Clearvue, and build a new blower cabinet to attach to the cyclone. Talked to Clearvue, they said you want the blower on top, and want a left rear intake on the cyclone, which the Aget has, so the flow of the air in the blower is the same as in the cyclone. Now the Aget dustkop has baffles in the tube coming up from the cyclone. I did some reading, and ran into a thread about a cyclone with baffles, and they were considered helpful to the cleaning of the air in the cyclone. So they will straighten the flow of air, and the direction of the air will not matter. These baffels fill the whole tube and reach almost to the bottom of the cyclone, and are angled like a arrow to the center of the cone. Was planning to take them out, but after reading that they may be helpful will leave them.

[Jim Andrew]

It would be interesting to watch a Clearvue cyclone in action. Looking at your drawing, it is hard to see the particles rotating around the edge of the cyclone. When I watch the hose coming out of my 2hp cyclone, the flow of dust rotates around the edge of the hose. So my idea is that the dust separates from the air around the edge, while the air flows up the tube.

[Matt Mattingley]

James, you pretty much hit this dead on, and I would have to say excellent description. The only thing I would like to add which was not 100% clarified, is the air does not change direction in it's circular motion, just it's elevation. In the example you used it has a motor top fan. The orientation direction of the fan being CCW the inlet and the outlet would have to work in the CCW direction as well to optimize efficiency.

I have played with, tweet, twisted and made adjustments in every shape and form to my homemade cyclone to get more bang. At 20 A all gates open, but I'm getting in closed system 20 inches WC and around 1800 CFM on one six-inch duct at longest run. Is there any room for improvement?

[James Gunning]

Jim,

In general, the airflow direction inside the cyclone and the airflow direction in the blower should match. I.E. they should both spin in the same direction. Reversing the airflow will cost in performance and HP needed to turn the impeller. Bill Pentz talks about this in his cyclone design discussion. If I were building or buying a blower, I would get one that turned in the same direction as the cyclone. However, it's not absolutely necessary in all cases. Ideally, the airflow up the central tube should not spin at all as it enters the blower impeller. There are some disadvantages to it spinning in either direction, so many professionally designed cyclones incorporate an air straightener inside the central tube to stop the spin. It sounds like your Aget cyclone incorporates an air straightener. Assuming that, you could turn your impeller in either direction and it should work the same. I've never seen a cyclone with an air straightener installed. If you can snap photos of yours, I would like to see what Aget put in there.

The only reservation I would have is if the air straightener caused clogs due to chips bypassing the cyclone and hanging up on it. I would think that's unlikely since little in the realm of chips bypasses the cyclone. The main cause of bypass is letting the chip bin get too full.

I'm planning to try an air straightener in my cyclone. I was just looking at how I could incorporate it.

[James Gunning]

To see a cyclone in action and better visualize the airflow, take a look at this video: https://www.youtube.com/watch?v=TJ_I8Sx8T38

[James Gunning]

Matt,

You are correct, the direction of the spin doesn't change. As air enters the cyclone it flows downward until about the bottom of the cone and than up into the central tube.

I'm not sure about your system design, and I'm far from a professional in that regard. Are you saying you are measuring 20" of static pressure in the ducting with all your gates closed? That seems like a high value, but if you are pulling an actual 1800 CFM at the end of your longest duct, I can't imagine how you would need more airflow.

[Matt Mattingley]

20" of water column in closed system pressure.

[Jim Andrew]

Have no idea how to post pics to this site. I can give measurements. The baffle is 8 x8, and goes from the top of the center tube down 36". The end at the bottom looks about 1 1/2 x 1 1/2", and tapers to the point at about 25" down. It is then 8x8 the rest of the way to the top. There are only 4 welds at the very top, holding the baffle in the pipe. Just measured this and wrote it down. Looks like the tube extends down into the cyclone 18". The total cyclone is 43" tall, the cone is 23" tall. The cyclone is 18" diameter.

[John K Jordan]

I installed a 5hp ClearVue in my shop and I can tell you it is entertaining to watch! For testing I had someone feed a variety of materials, dust, chunks, bark, plastic shavings, etc. Mine is mounted in a noise containment closet so I can't watch it from the shop.

JKJ

[Jim Andrew]

It would be interesting to feed some smoke into the clearvue to see how the air flows.

[James Gunning]

Jim,

Please forgive the crude sketch. It's my interpretation of your description, drawn on a 3x5 card. What I understand is this: the center tube on your cyclone is 8" in diameter and 18" long. The baffle (air straightener) is overall 36" long. It sticks out of the bottom of the center tube about 18" and tapers from about the 25" point from the top to a width of 1.5". I'm assuming it is in the form of a X inside the tube. How did I do? Way off or in the ballpark?

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[Jim Andrew]

You got it. Now draw the cyclone body. 18" diameter, 23" tall cone, 43" total height.

[Steve Peterson]

I installed a 5hp ClearVue in my shop and I can tell you it is entertaining to watch! For testing I had someone feed a variety of materials, dust, chunks, bark, plastic shavings, etc. Mine is mounted in a noise containment closet so I can't watch it from the shop.

JKJ

They are cool to watch, but the sound gets to you after a while. A sound closet is almost a requirement. I have thought about adding a video camera to be able to watch it all the time.

Steve